Geology

Octopus Spring is 8 miles North of Old Faithful geyser near the Paint Pot area, inside the most recent caldera of Yellowstone National Park, left by an eruption 630,000 years ago. The water of the spring is warmed by the heat from the magma below and percolates through rhyolite, a silica rich volcanic rock. The hot water dissolves the silica. When it reaches the surface, the silica precipitates from solution, resulting in grayish white silicate deposits called sinter.

Geochemistry

The temperature at the source of the spring can be as high as 92° Celsius and slowly declines to the lower 60’s away from the spring source. When we measured the temperature with an infrared thermometer it was 84° Celsius at the source surface. This temperature gradient in the outflow results in the varied microbial life in this system. There is a lack of hydrogen sulfide in the water, resulting in an alkaline system with a pH around 8.4 as measured at the source. We know that hydrogen gas is in the water due to the presence of bacteria that eats it, which occurs in the outflow of the spring.

Microbial Life

Octopus Spring is rich in microbial life because pH and the presence of hydrogen sulfide are not limiting factors in relation to other hot spring systems in the park.

Aquifex, a pinkish-purple hydrogen-eating chemotroph, which can live in temperatures up to 88° C. Aquifex is found close to the source of hot water, beginning approximately 2 meters away from the spring. Chemotrophs use chemicals as a source of energy; Aquifex uses oxygen from the air to oxidize hydrogen gas that is present in the spring water.

As the water flows away from the spring, it cools, allowing for cyanobacteria like the vivid green Synechococcus, to grow. Cyanobacteria are aquatic photosynthesizing bacteria. Synechococcus has the highest temperature tolerance of the cyanobacteria in this ecosystem. Where the water cools below about 72° C these microorganisms are present to use oxygenic photosynthesis (with water and sunlight), similar to photosynthesis in plants. In this area Synechococcus occurs in microbial mats, overlaying Chloroflexus, a green anoxygenic photosynthetic cyanobacteria.

The diversity of the microorganisms increases as the temperature decreases to 62° at the end of the stream. Phormidium, a filamentous cyanobacteria, is the visible microorganism with an orange coloring. As the temperatures cool, the Phormidium gives way to a blackish cyanobacteria called Calothrix. Calothrix forms the top layer of microbial mats as shown in the diagram below. It is an oxygenic photosynthetic cyanobacteria which uses photosynthesis during the day and fermentation at night. The waste product of this fermentation feeds the next layer of Chloroflexus-like anoxygenic photosynthesizers. Unlike the top layer, these organisms use the deep red and infrared light which passes through and is unused by the cyanobacteria. The bottom layer is the chemotrophic microbial community, which utilizes the organic matter trickling down from the layers above. The anoxygenic photosynthesizers and the chemotrophes may also use the hydrogen filtering down in the water for energy.

Cross section of microbial mat (not to scale). The vertical layering results from different microbes having different requirements for light and chemistry. An upper microbial community might shield the organisms below from intense sunlight.

Cross-section of a microbial mat from Octopus Springs. The top green layer is comprised of the cyanobacteria, Synechococcus; the mat below contains the filamentous green cyanobacteria, Chloroflexus.
Hi-Resolution Image

Left, illustration of the stream flowing downward with the colorful microbial organisms. Above, the bottom of the stream leading into the mat.

Evidence on Other Planets

In the May 21, 2007 NASA Press release, the Spirit Rover, while traveling backwards across the Gusev Crater, drug its locked wheel, uncovering a white rock layer under the dust. When x-ray spectroscopy looked at this rock, the data showed that the rock was 90% silica. Scientists believe that this kind of rock could have been deposited by a volcanic vent or a hot spring; similar to the low sulfur area of Octopus Spring in Yellowstone National Park. By looking closer at sites like these on Mars and elsewhere, we might be able to see indirect evidence of microbes encased in the silica precipitated in these areas. While viewing the dry versions of these environments on Earth with the tools we have on the rovers on Mars, we might be able to greatly increase our understanding of the information our rovers can get millions of miles from Earth.